The present invention relates to an interference film filter used such as for an optical multiplexer and demultiplexer in optical communication systems and optical microscopes and to an optical waveguide used such as for optical measurement systems and optical integrated circuits and relates to a method for producing the interference film filter and the optical waveguide.
Japanese Patent Laid-Open No. 60-87302 (1985) discloses an interference film filter including alternative lamination of a thin Ti0.sub.2 film having a high refractive index of 2.2 and a thin Si0.sub.2 film having a low refractive index of 1.46.
Japanese Patent Laid-Open No. 60-114811 (1985) (U.S. Ser. No. 06/674,770, filed on Nov. 26, 1984 and assigned to the present assignee , now U.S. Pat. No. 4,737,015 disclosed a stepped and a graded optical waveguide including an oxynitride film composed of silicon oxide and silicon nitride having a high refractive index of 1.72 at a wavelength of 0.633 .mu.m, the wavelength generated by He-Ne gas laser and forming a core portion and an oxynitride film composed of silicon oxide and silicon nitride having a low refractive index of 1.65-1.69 at a wavelength of 0.633 .mu.m and forming a clad portion. The oxynitride films are deposition-formed by sputtering a polycrystalline silicon in a sputter gas containing 0.sub.2 and N.sub.2 gases, and refractive indexes of the oxynitride films are controlled by adjusting the 0.sub.2 gas content in the sputter gas.
Japanese Patent Laid-Open No. 60-37504 (1985) discloses a stepped and a graded optical waveguide comprising a core portion formed of an alternate lamination of a thin Ti0.sub.2 film having a high refractive index of 2.3 and a thin Si0.sub.2 film having a low refractive index of 1.477 at a wavelength of 0.633 .mu.m. The laminate is then heat-treated to form a core portion having a uniform composition and a refractive index between 1.477 and 2.3.
In the interference film filter and the optical waveguide, when a thin film having a high refractive index n with thickness d exhibits a local maximum transmittance at a wavelength .lambda., the following equation generally holds; EQU d=.lambda./4n(2m+1)(m=0,1,2,3, . . . )
The above equation indicates that the film thickness be decreased with the increase in refractive index of the film. On one hand, loss of light in general decreases with the decrease in thickness of the film.
Loss of light in the interference film filter and the optical waveguide is further reduced in dependence upon extinction coefficient specific to a material of thin film. The extinction coefficient represents a degree of light absorption in the thin film material, in that, the smaller the extinction coefficient, the smaller the loss of light in the thin film. Until now, it was considered that the extinction coefficient of an optical material in general increases in dependence upon an increase of the refractive index thereof.
Refractive index and extinction coefficient of the conventionally used Ti0.sub.2 thin film for the interference film filter and the optical waveguide are respectively 2.3 and 0.5-1.0, which are not necessarily satisfactory for the reduction of loss of light in the interference film filter and the optical waveguide.